1. Field of the Invention
The present invention relates to a photomask for forming holes in making a semiconductor device at the exposure step and a process of making a semiconductor device by the use of the photomask.
2. Description of the Related Art
Recently, a process rule for semiconductor device is increasingly refined, accompanied by a number of new techniques. In the photographing step, that portion of the process until the process rule on the level of submicron has been constructed in accordance with the g-ray exposure technique. However, another exposure technique using an i-ray of shorter wavelength is now being introduced accompanied by the shift of the process to halfmicron. Further, the developing step takes a post-exposure bake for purposes of reducing the standing waves and so on.
On advancing the refinement of the process rule for semiconductor device, it becomes impossible to neglect the diffraction of the exposure light. It is thus required to provide a photomask dealing with the diffraction.
FIGS. 10A to 10G are diagrammatic views illustrating a conventional pattern of resist to be produced, a photomask for the resist pattern while dealing with the diffraction and patterns of resist formed by the use of the above photomask, respectively. The resist pattern of FIG. 10A includes a central hole and four corner holes formed therein. In order to obtain such a hole pattern, such a photomask as shown in FIG. 10B is used while considering the diffraction of light. More particularly, the photomask includes five square holes corresponding to the five holes in the resist pattern, these five square holes being intended to provide circular holes in the resist due to the fact that the light passed through the square holes in the photomask is diffracted.
Where a spherical lens is used in an optical stepper, however, the aforementioned photomask is undesirably influenced by an aberration on exposure. Exact hole patterns may not be obtained by the photomask. More particularly, each of the corner holes in the resist of FIG. 10C may be formed to be of an elliptical configuration extending radially from the center of the resist. This may adversely affect various characteristics of the semiconductor device produced.
The degree of integration in the semiconductor device can be increased by the refinement of the process rule for semiconductor device, and the size of chip of the semiconductor device tends to be increased. In order to make a chip of such an increased size, the optical stepper will be required to have a broader exposure area. If the mass-production is considered, similar requirements are also in the process rule of the prior art.
In order to enlarge the exposure area of the optical stepper, it requires an optical system using a lens assembly of larger aperture, accompanied by various aberrations that are created in the resist at the corners and marginal edges thereof.
In general, the problem of aberration can sharply be overcome if the aperture of a lens assembly used in the reduction exposure device is very largely increased while only using the central portion of such a lens assembly. In the reduction exposure device, however, the magnitude of the lens assembly is limited. Therefore, the aberration was intended to be reduced by utilizing a combination of convex and concave lenses as in the general optical system. However, a manner of combining these lenses is also limited. The prior art cannot necessarily eliminate the influence of the aberration.
Accordingly, the prior art provided a resist having holes of forms very different from the circles at the regions of the resist corresponding to the marginal edge of the lens. The precision is deteriorating at the hole portions in the etching, post-etching CVD or spatter step. This may adversely affect various characteristics of the semiconductor device.